The present invention relates to a system and a means for interconnecting an operation of flow lines, particularly in connection with submerged pipelines and riser conduits. The invention relates further to methods for establishing such systems.
As basis for intercoupling submerged and bottom based pipelines and other conduits or riser pipes are today used so called templates or bottom frames, which among other things can serve as stations for coupling the pipelines together. The templates may be equipped with pipe couplings, various types of valves and other production equipment. In addition the templates serve as a fundament for positioning different types of tools which may be used for pull-in operations and installation of the pipelines.
The bottom based pipelines in such systems have frequently substantial dimensions such as 20-50 centimeters in diameter or more with corresponding high weight and large stiffness. The pipelines may extend several kilometers along the sea bottom. In order to carry out the pull-in and coupling on such pipelines on to an usually already installed template, large and usually heavy powerful tools including winch equipment are required, and which during the operations must be positioned and mounted in stable fashion on the template. Bottom frames of this type assume therefore considerable dimensions, and may with the installed equipment assume weights up to 200 tons or more. The pull-in equipment may likewise frequently assume a weight of 25-35 tons and must provide pulling forces in the range 30-80 tons.
Bottom frames are usually piled to the sea bottom or are provided with skirts in order to sustain horizontal forces, both in connection with the hauling operations and from the thermal loads and stretches arising in the pipelines during the transport of oil and gas.
When the coupling of bottom bases pipelines have been accomplished, and the bottom based transport station is put into operation, large problems may arise in connection with the thermal movements in the pipelines when this is used for transport of oil and/or gas with temperature which may vary from ambient temperature up to for instance 70-80.degree. C. The theoretical elongation in a pipeline of actual type with a length of 1-2 kilometers may be 1-2 meters. In order to distribute movements and forces, the pipeline is usually arranged with curves along the sea bottom such that thermal expansion and contractions may take place laterally, but the final or end part of the pipeline facing the bottom frame must necessarily extend in a straight line towards the pipe coupling. Since the bottom frame is completely restrained from movement, the coupling will be subjected to large stresses due to thermal movements, not only tensional and compression stresses, but to more critical bending stresses. Such bending stresses in the pipeline will increase progressively towards the coupling itself, and will reach a peak point therein. In order to reduce the risk for failure in the coupling, the coupling units must be over-dimensioned relative to the pipe.
The principal object for the present invention is to provide a new system solution to be used in connection with intercoupling of subsea pipelines particularly between subsea production pipelines and riser pipes. A further object of the invention is simultaneously to develop coupling equipment having lower weight, a factor which will provide substantial technical and economical advantages.
With the designation "pipeline" shall herein be understood various types of pipe bundles or mantles encompassing two or more pipes, cables or the like.
The invention is based upon a new concept, namely to arrange the coupling system displaceable on the sea bottom during the hauling and coupling operations as well as later when the system is put into operation. The invention is more particularly characterised in that a bottom frame including pipe couplings, valves and other equipment are being mounted on a relatively light weight sledge-like structure which is adapted for displaceable support on the sea bottom or optionally one on the sea bottom positioned bottom plate, fundament or the like conforming to the character of the sea bottom.
A further important feature of the concept of the invention is that it makes it possible to use relatively light weight hauling- and coupling equipment because instead of pulling the end of the pipeline resting on the sea bottom to the bottom frame, the bottom frame may be pulled to the pipeline, an operation which in most cases will require far less power, and simplify the coupling operation. When the coupling between the pipeline and the displaceable bottom frame is finalised, a riser pipe, flexible or not, may be coupled at the other side or end of the bottom frame. A riser pipe will normally rest on the sea bottom along a certain stretch. This implies that when the system is put into use, all thermal movements in the pipeline will be transferred to the displaceable bottom frame and further to the riser pipe, which however without large problems can move forward and backwards on the sea bottom along the stretch or section resting on the sea bottom.
It is priority known to use belt vehicles and sledge-like structures for towing and transporting pipelines along a sea bottom. As illustrative for the prior art one can refer to U.S. Pat. Nos. 4,457,378, 4,525,101 and 4,793,737.
U.S. Pat. No. 4,525,101 shows a pipeline with the end section mounted on a sledge which can be hauled along the sea bottom by means of hauling equipment mounted on submerged tool which is suspended in a cable down from a sea surface vessel. The pipeline shall be coupled to a second pipeline, which likewise is mounted on a sledge-like stretcher.
U.S. Pat. No. 4,793,737 shows a pipe coupling station mounted on a sledge. A flexible riser pipe is submerged down from a sea surface vessel and coupled to the station, whereafter the unit is pulled to a fixed mounted bottom frame by means of winch equipment mounted on the bottom frame in order to couple the riser pipe to production equipment on the bottom frame.
U.S. Pat. No. 4,457,378 shows a pipe coupling module mounted on a sledge designed to be directly coupled to a bottom based pipeline. The coupling station is coupled to the pipeline, either at the sea surface or at the bottom. In the vicinity of the pipeline is on the sea bottom positioned a bottom frame. Upon this frame is submerged a second module with winch equipment, thereafter the coupling module including the pipeline is winched into the bottom frame and is elevated up from the sea bottom to a position on the bottom frame by means of coupling and winch equipment which is lowered down from a sea surface vessel.
The preceding briefly described technique has in common that the sledge structures are utilised for transporting sledges together with a pipeline in order to pull the before mentioned equipment to a fixed stationary bottom frame by means of winch equipment mounted thereon.
There is further known a coupling solution which in the oil industry frequently is named as "PLEM" (Pipeline End Manifold). The essence of this solution is that a pipe end structure is being welded to a pipeline in position on the sea surface, where after the unit including the pipe is lowered down on to the sea bottom for later coupling to a second pipeline or riser pipe.